I ordered an adjustable 24V to 5V DC-DC step-down unit from eBay for a few pounds, searching specifically for a KIS-3R33s based unit based on advice I'd read online. The claimed specifications for anyone wanting to buy something similar were:

IS-3R33S integrated with adjustable MOSFET MP2307DN

Size: 45mm by 32mm

Input voltage: 4.75-24V

Output voltage: 0.93V-18V

Efficiency: up to 98%

Output current: rated 3A, max up to 4A peak

Programmable Soft-Start

Low ESR Ceramic capacitors is recommended at output

Internal Frequency: Fixed 340kHz

Output ripple: 20M bandwidth (for reference only)

Protection: Cycle-by-cycle over current protection.

The rubbery coating on mine didn't look too good, but it works OK. Notice the small screw on the blue block between the large orange capacitors - that controls the output voltage:

KIS-3R33S based DC step-down

I attached this to a couple more 2.1mm barrel connectors from Maplins for ease of use - given that's what the rest of my setup uses:

Surprisingly Maplins didn't have any 24V DC supplies in store, so again I turned to eBay and ordered a 24V 1200mA power supply (aiming for a reasonable power rating), which was the last part to arrive.

Initially I set the DV output to 5.25V to match what the original power supply I'd used gave. That worked but under load this fell to just 4.8V, so by fine tuning the DC step-down converter I could ensure the the Raspberry Pi was getting the full 5V it wants even with the two web cameras attached.

The observant may wonder why the green LEDs on the Xbox Live Vision web cameras are not on - that's because I've disabled them for astro-photography and use in a bird nesting box (the camera on the right has four infra-red LEDs added to it).

My next project will be to install the Raspberry Pi and voltage step-down module in a suitably water tight container to mount in the garden, and run the 40m ethernet cable to it. Then I should finally be able to monitor the bird box remotely.

Using powered USB hub is probably needed if you want to connect lots of power hung devices at once. However, using a good power supply directly connected to the Raspberry Pi as usual, there is no problem running two of the web cameras and using the wired ethernet connection (which is also USB internally) - but I do have nothing else connected, no sound, no video, no data pins, etc. In my experiments at least, this is also possible sending the power via this kind of basic POE cable.

Thank you for the article I'm interested in doing this can you take a photo of the PSU and show how it's connected to the Ethernet cable or maybe just explain in the comments? Also if you can provide a link to the PSU that would be great